Within the field of portable radio devices there is commonly a need to make these devices operational at several frequency bands. Typically, portable radio devices are small and usually there is a limited space for providing this operational capacity.
The antenna arrangement in particular has turned out to be a crucial factor. Basically, different frequency bands require separate antennas which may not fit in the limited space of a portable device. Therefore, a single wideband antenna has frequently been used in portable radio devices.
However, it is a difficult task to design a single antenna small enough to fit in a portable device and efficient enough to provide a high performance over several different frequency bands. One approach has been to utilize the fundamental principles of a so-called monopole. As is well known, a monopole is basically a half dipole.
A typical dipole antenna 100 is schematically illustrated in FIG. 1. The typical dipole antenna 100 includes two feed lines 110, 110′. An end portion of each feed line 110, 110′ is bent in a substantially perpendicularly direction with respect to feed line 110, 110′ so as to form two antenna elements 112, 112′. A length of each antenna element 112, 112′ is approximately one-quarter of the wavelength at the resonant frequency f0 (e.g., λ/4, where λ is the wavelength of the resonant frequency f0). In other words, a total length of the antenna elements 112, 112′ is about one half of the wavelength at the resonant frequency f0 (e.g., λ/2). Dipole antenna 100 is typically operated at a single frequency f0.
To make the conventional dipole antenna more compact, a simplification of the antenna can be formed on a suitable substrate arrangement (e.g., a circuit board or a similar device). This is schematically illustrated in FIG. 2a and in FIG. 2b. FIG. 2a presents a top view a monopole antenna 200, and FIG. 2b presents a cross-section of monopole antenna 200 in FIG. 2a, as seen in a direction indicated by the arrows A-A.
Monopole antenna arrangement 200 in FIGS. 2a and 2b includes a substrate 250 (preferably a dielectric substrate), an electrically conductive patch line 210 (preferably a metallic patch line), and a ground metal plate (or ground plane) 220 formed on a top surface of dielectric substrate 250 at the same side as patch line 210. Alternatively, ground plane 220 may be formed on a bottom surface of the dielectric substrate 250, or in dielectric substrate 250. One end of patch line 210 is formed as a signal feed point 230, whereas another end of patch line 210 is formed as an antenna element 212 having an L-shape so that antenna element 212 extends from ground plane 220 in a direction substantially perpendicular to patch line 210. Monopole antenna arrangement 200 is formed by antenna element 212 interacting with ground plane 220.
Monopole antenna arrangement 200 takes advantage of ground plane 220 and well known image theory to map patch line 210 and the inverted L-shaped antenna element 212 so as to form a fictive second antenna element 212′, as indicated by dashed lines in FIG. 2a. As a result, monopole antenna arrangement 200 having antenna elements 212, 212′ substantially equivalent to antenna elements 112, 112′ of dipole antenna arrangement 100 is formed. Monopole antenna arrangement 200 is typically operated at a single frequency.
Even if the fundamental principles of monopoles may be used to accomplish an antenna that is smaller than a full dipole antenna, it is still only suitable to operate in one frequency band.